Two people share a home. One of them has a severe, permanent mental impairment, certified by a doctor. Does the household get a 25% discount off its council tax, or does the bill disappear completely? The answer turns entirely on how many of the residents meet the legal test — and the gap between the two outcomes is a clean, constant multiple in every council tax band, in every local authority in England.
What "severely mentally impaired" means for council tax
The disregard sits in Schedule 1 to the Local Government Finance Act 1992, which lists the categories of resident a council must leave out when it counts how many adults live in a property for discount purposes. For the severely mentally impaired category, two things both have to be true on the day in question: a registered medical practitioner has certified that the person has a severe impairment of intelligence and social functioning, however caused, which appears to be permanent; and the person is separately entitled to one of the qualifying disability-related benefits set out by the Secretary of State. Meeting only one of the two conditions is not enough, and it isn't a self-assessment — the certificate and the qualifying benefit both have to be confirmed with the council before the disregard applies.
One qualifying resident: treated as living alone
Where a two-adult household has exactly one SMI-qualifying resident, that person is disregarded completely when the council counts occupants. A property with one disregarded adult and one ordinary adult is then billed exactly as if a single person lived there — the standard 25% Single Person Discount under section 11 of the 1992 Act, the same discount this site examined for a different pairing in stacking the single person discount with the Disabled Band Reduction Scheme.
Both residents qualifying: the bill goes to zero
Where both adults in the household meet the SMI test, there is no one left in the property who counts as a liable, non-disregarded occupier at all. Council tax law doesn't stretch the discount further in that situation — it exempts the dwelling outright. Class U of the Council Tax (Exempt Dwellings) Order 1992 covers a dwelling occupied only by one or more severely mentally impaired people, or by SMI people together with certain other disregarded occupants such as full-time students. Where it applies, the annual bill is reduced to nothing, not to 75% of the standard charge.
A third, more common situation sits between these two: a household where one resident is SMI-qualifying and another resident is present but doesn't meet any disregard category — a partner who doesn't have the certification, an adult child, a lodger. That household gets the ordinary 25%-equivalent discount described above, not the exemption, because a non-disregarded adult is still living there. Working out which of the three situations applies is a job for the council's benefits team, not a guess from the household.
The cash difference, band by band
Applying the statutory A-H band ratios (section 5 of the 1992 Act: Band A is six-ninths of the Band D figure, Band H is eighteen-ninths) to the national mean 2026-27 Band D charge of £2,410.97 shows the same pattern repeating at every band:
| Band | Full annual bill | One SMI resident (25% off) | Saving | Two SMI residents (exempt) | Saving |
|---|---|---|---|---|---|
| A | £1,607.31 | £1,205.48 | £401.83 | £0.00 | £1,607.31 |
| B | £1,875.20 | £1,406.40 | £468.80 | £0.00 | £1,875.20 |
| C | £2,143.08 | £1,607.31 | £535.77 | £0.00 | £2,143.08 |
| D | £2,410.97 | £1,808.23 | £602.74 | £0.00 | £2,410.97 |
| E | £2,946.74 | £2,210.06 | £736.69 | £0.00 | £2,946.74 |
| F | £3,482.51 | £2,611.88 | £870.63 | £0.00 | £3,482.51 |
| G | £4,018.28 | £3,013.71 | £1,004.57 | £0.00 | £4,018.28 |
| H | £4,821.94 | £3,616.46 | £1,205.48 | £0.00 | £4,821.94 |
Look down the two saving columns and the relationship is exactly fourfold at every single band — £1,607.31 is precisely four times £401.83, and £4,821.94 is precisely four times £1,205.48. That isn't a coincidence specific to the national mean: because both outcomes are a straight percentage of the same underlying bill (25% against 100%), the ratio between them is 100 ÷ 25 = 4, and it holds regardless of band or council. That's a tidier relationship than this site found when it stacked the single person discount with the Disabled Band Reduction Scheme, where mixing a flat banding-step reduction with a percentage discount produced a multiple that moved between 2.13x and 3.00x depending on the band — a reminder that a constant ratio should never be assumed, only computed.
The same fourfold gap holds at the extremes of England's council tax range. At Band D, Wandsworth — England's cheapest authority for 2026-27 — charges £1,028.21: the one-resident discount saves £257.05, the two-resident exemption saves the full £1,028.21. Dorset, the most expensive, charges £2,765.02 at Band D: the discount saves £691.26, the exemption saves £2,765.02. Four times the saving, in the cheapest council and the dearest, because the 4x relationship is arithmetic, not geography.
| Band D, 2026-27 | Full bill | Discount saving (1 qualifying resident) | Exemption saving (2 qualifying residents) |
|---|---|---|---|
| Wandsworth (cheapest) | £1,028.21 | £257.05 | £1,028.21 |
| National mean | £2,410.97 | £602.74 | £2,410.97 |
| Dorset (dearest) | £2,765.02 | £691.26 | £2,765.02 |
Not the same mechanism as the Disabled Band Reduction Scheme
It's worth being precise about the difference from the Disabled Band Reduction Scheme, which this site has covered separately. The Disabled Band Reduction Scheme is about the property — it applies where a home has been adapted to meet the needs of a disabled resident (an extra bathroom, a wheelchair-width door, specific indoor space for equipment) and moves the whole property down one council tax band before any discount is calculated. The severely mentally impaired disregard covered here is about the person — it depends on who lives in the property and whether they meet the medical and benefit tests, and it changes which discount or exemption class a household falls into, not which band the property sits in. A property and a resident could in principle qualify under both tests at once, but that combination raises its own questions and isn't covered by the figures above. For the fundamentals of how the eight bands themselves are set, see how council tax bands work.
Checking eligibility
Both the discount and the exemption have to be applied for — a council does not back-apply either automatically. The process typically needs a medical certificate from the person's GP or consultant confirming the impairment and its likely permanence, together with evidence of the qualifying benefit, submitted to the council's council tax team. Backdating is often possible but depends on the individual council's practice and how long the qualifying conditions have applied. This is general information about how the rules work, not advice on any individual household's position — speak to your local council's council tax team or a qualified welfare rights adviser before applying.
Source data: 2026-27 council tax Band D charges across 296 English local authorities (gov.uk/MHCLG), fetched 2026-10-12. See more Homecost guides.