Loft conversion

[AdamD]

Currently have a 3 bed semi detached with two decent sized bedrooms and a box room converted to home office. Have a toddler and baby so need a solution to give them their own rooms and a loft conversion seems the way forward. To move to a bigger house in the area will be another £150k so is out of the question. We have a good loft space and could make it the biggest room plus en suite. Now, budget. No idea really how to forecast this as totally new to it but it would all need to be done by trades. All in company would be best as we want minimal hassle and want it done as quickly as possible, won’t be easy with a 2 year old and a 5 month old baby, but appreciate this will be far more expensive. What we want is a rear dorma and hip to gable at the side for maximum space and headroom. 35 sq metres roughly. We are on the south coast fyi.

What sort of ball park budget should be realistic and what other considerations before proceeding?

 

 

[Redbeard]

No idea about the price but 'second-hand' experience tells me not to assume that not all 'loft conversion specialists' do a fantastic job. Issues range from failing to understand that the party walls of a terraced house attic (with a cold loft on either side) need insulating, not just dot-and-dab plasterboard, to dreadful detailing of insulation and air-tightness and a complete failure to calculate properly the eaves ventilation needed. 

1 hour ago, AdamD said:

what other considerations before proceeding?

 

Well, if the FF ceiling joists are typical and the space above the joists is not as generous as you think once the 'joist uplift' and insulation 'downward thrust' are taken into account you may have to take the FF ceiling level *down*, which takes your FF out of use for the duration.

[AdamD]

Sorry, what is FF?

[Redbeard]

Sorry! First Floor. The joists immediately below the space you hope to use.

Edited 11 hours ago by Redbeard

[AdamD]

Thanks. I’m a layman so not massively following, whilst I kind of understand what you’re saying. What is typical and how much space may be lost ?

 

can you explain joist uplift and downward thrust in this context please ?

[ETC]

Is your house timber framed?

[Redbeard]

OK, let's say you currently have 75mm (quite typical) deep joists and your structural engineer says you need 175 or 200mm joists. Your new joists will either protrude above (reducing the height in the new bedroom) or below the joists (reducing the ceiling height in the room below) by 100-125mm. Let's assume that you have 75mm rafters too. The gov't's guidance suggests a 50mm ventilation gap above any insulation, so you can only get 25mm between the 75mm joists. If you use polyisocyanurate (PIR) insulation (Kingspan or Celotex) you'll need a further 125mm to get a compliant U value of 0.16W/m2K as required by the Building Regulations. Add to that the need for say 25mm battens and 15mm of plasterboard and skim, and the need to to have some ventilation 'crossover' at the top of the room (and of course the need for floorboards which I have not yet advised you to put on top of the joists(!) ) and you'll see how the height loss builds up. If, like many attic rooms, you want to have a flat area of ceiling at the top of the room you will of course reduce the room height still further. If we go for 2.1m as a desirable minimum height the example above (losses at top and bottom of the room) could mean that you lose up to 290-315mm off the apparently-available height. If you decide to use more 'friendly' insulants you'll lose more than that. If you have a good tall steep-pitched roof then you may be fine. Not tall and not steep-pitched, you may be less fine. 

Edited 8 hours ago by Redbeard

[AdamD]

1 hour ago, Redbeard said:

OK, let's say you currently have 75mm (quite typical) deep joists and your structural engineer says you need 175 or 200mm joists. Your new joists will either protrude above (reducing the height in the new bedroom) or below the joists (reducing the ceiling height in the room below) by 100-125mm. Let's assume that you have 75mm rafters too. The gov't's guidance suggests a 50mm ventilation gap above any insulation, so you can only get 25mm between the 75mm joists. If you use polyisocyanurate (PIR) insulation (Kingspan or Celotex) you'll need a further 125mm to get a compliant U value of 0.16W/m2K as required by the Building Regulations. Add to that the need for say 25mm battens and 15mm of plasterboard and skim, and the need to to have some ventilation 'crossover' at the top of the room (and of course the need for floorboards which I have not yet advised you to put on top of the joists(!) ) and you'll see how the height loss builds up. If, like many attic rooms, you want to have a flat area of ceiling at the top of the room you will of course reduce the room height still further. If we go for 2.1m as a desirable minimum height the example above (losses at top and bottom of the room) could mean that you lose up to 290-315mm off the apparently-available height. If you decide to use more 'friendly' insulants you'll lose more than that. If you have a good tall steep-pitched roof then you may be fine. Not tall and not steep-pitched, you may be less fine. 

Really interesting and useful, thanks. I’ll challenge the builders with this when I speak to them about the quotes.

 

not sure what is classes as tall and steep so I’ll get up there tomorrow and measure to, say, the highest point and post here for opinions!

[Redbeard]

I did not complete the sum! My dodgy maths above suggest that if you do not drop the joists into the room below you'd need at least 2200- 2300 height from the existing joists to wherever the ceiling would want to be. That would give you a very 'pointy' room with little or no flat ceiling, so it would 'feel' better if there was more flat ceiling, which of course means more overall height required.

[Redbeard]

9 hours ago, AdamD said:

what other considerations before proceeding?

Check if this is Permitted Development or needs Planning Permission. You may already have done that.

[joe90]

You only need planning permission if installing a dormer in the front facing roof, dormers on the rear are permitted development (tho the council may tell you you need planning permit ion!!,!). Veluxs on the front do not need planning permission. You WILL need building regs whatever you do. I would suggest you get multiple quotes from loft conversion companies with personal recommendations not reviews which companies can write themselves. If companies cannot give you previous customers details to get your own feedback avoid them. 

[olley]

I got a quote for our dormer conversion this year. 7m x 10m bungalow with full length dormer.

Quote was for 2 beds and 2 bathrooms ready for occupancy, £125,000.

Just for a bare shell ready for me to insulate, second fixings, plumbing etc I was quoted over £60,000